Two distinct CD3-associated T cell receptors, TCRalphabeta and TCRgammadelta are found expressed on T lymphocytes. alphabeta T cells recognize foreign and self peptides presented by Class I and Class II MHC molecules. They typically use CD4 or CD8 molecules as co-receptors for MHC/peptide recognition nd express a variety of Valpha and Vbeta gene segments. In contrast, recognition by gammadelta T cells is not well understood. Although fully functional, few gammadelta T cells have been demonstrated to respond to peptide antigens in a MHC restricted fashion and most do not express CD4 or CD8. Finally, gammadelta T cells use only a few Vgamma and Vdelta T cells expand in patients with a number of infections (up to 45% of all T cells int he peripheral blood), they are likely to be involved in the host response to infection. Moreover, an increase in gammadelta T cells has been seen in the lungs of patients with sarcoidosis, the synovium of patients with rheumatoid arthritis, and in acute plaques in the brains of patients with multiple sclerosis. We have made a major breakthrough in elucidating one type of human gammadelta Tcell recognition by defining synthetic, nonpeptide, monoalkyl phosphate compounds as antigens / ligands for T cells expressing the Vgamma2 and Vdelta2 gene segments. The chemically similar natural ligands produced by mycobacteria and other bacteria stimulate gammadelta T cells at a very high frequency (1 out of 2-19 cells). This responses dependent on the expression of the Vgamma2Vdelta2 TCR as demonstrated by our ability to transfer reactivity to recipient TCR-T cells by transection of a Vgamma2Vdelta2TCR. Here, we propose to extend these observations. In Aim 1, we will delineate the TCRgammadelta domains that mediate the recognition of the nonpeptide ligands by extending our transfection studies to include different TCR V genes and mutated TCR V genes. In Aim 2, we will identify the antigen presenting molecule for the nonpeptide ligand by characterizing candidate mAbs that specifically block gammadelta recognition of these ligands. In Aim 3, we will complete our purification and structural identification of the major myobacterial nonpeptide ligand. In Aim 4, we will determine the in vivo effects of nonpeptide ligands in rhesus monkeys. These basic studies to define the nature of antigen recognition by human gammadelta T cells should clarify their role in immunity and autoimmunity.